Tapering the battens changes their flexibility. By changing the batten flexibility, you can change the position and depth of the draft in the sail. However, you're limited by the shape that's cut into the sail to begin with. Small changes are possible - big ones, not so much.

The top battens on the 16's main are so short, they need to be tapered just to be able to bend enough.

On the lower battens, the draft adjustment is done with batten tension. More tension = more draft.

Typically, you want the forward 1/3 of the batten to be more flexible than the back 2/3.

Some racers taper their jib battens to make them softer and give the sail a little more shape.